The present invention refers to an apparatus for measuring visual performance for ophthalmologic examinations. Observation of the visual acuity of both eyes, of a subject being examined, is the basic procedure of every ophthalmologic examination aimed at quantitatively assessing visual performance and the extent of any refractive correction necessary to optimise the functionality of the vision apparatus.
Such observation is normally performed by covering one eye at a time, of the subject being examined, and sequentially subjecting the individual to a series of characters and/or graphic signs of progressively smaller size; the patient is asked to recognise the characteristics of each character and/or figure shown, until the discrimination limit is reached. Since each character size corresponds to a progressive value, increasing with a decrease in size, normally expressed as a fraction (e.g. {fraction (4/10)} or {fraction (20/50)} or {fraction (6/15)}) or as a decimal (e.g. 0.4), the value corresponding to the smallest character sizes correctly recognised corresponds to the visual acuity of the eye being examined.
Measurement of visual acuity is usually performed in mesopic illumination conditions (semi-darkness) and the characters to be recognised are shown at a typical distance of 4-6 meters from the subject; this can be with the naked eye, if emmetropic (i.e. uncorrected visual acuity) or with corrective ocular prostheses worn (spectacles or contact lenses) and, hence, concerns visual acuity with correction.
The characters to be recognised may be displayed on externally illuminated signs or on translucent backlit panels (optotypes in both cases) or visualised on an opaque screen through a light projector; the characters are generally black on a transparent background and normally include stylized capital letters of the alphabet, and arranged on a single square grid. Actually, various print style and selection standards of the same letters exist, to produce a set of characters of equal legibility (e.g. Snellen or Sloan).
Numbers can also be used in addition to letters, while other standards use a single character (e.g. Albini""s E or Landolt""s broken C test), the orientation of which is varied (top, bottom, right, left): these tests are meant for illiterate individuals, namely pre-school children.
The character size may, in turn, follow a linear or geometrical progression, with different ratio values between successive terms; the most widespread scale is logMAR, in which the ratio is 1010≈1.259.
A further visual functionality examination of considerable importance is the contrast sensitivity test. Once again this is performed by covering one eye at a time and presenting the patient with a sequence of figures characterised by black and white linear monochromatic reticula.
The reticula are of two types: square and sinusoidal wave, i.e. their brightness outline varies perpendicularly, like a square or sinusoidal wave, to the direction of the reticulum lines.
Within the sequence, the reticula vary both in terms of spatial frequency (number of cycles per view angle unit subtended by the observation point of the patient, as well as contrast (defined as the ratio between the difference: maximum brightness-minimum brightness and their combined total); the visibility of the reticula decreases with an increase in the spatial frequency and a decrease in contrast.
Of current use in clinical tests are reticula with a frequency of 1-30 cycles per grade and contrasts between 100% and 10%; they are usually made of shapes printed on card or glass (backlit in this case), to be observed in photopic conditions from a typical distance of 2 meters.
The subject being examined is invited to recognise the orientation of each reticulum of the sequence and, for each spatial frequency, the lowest contrast value correctly recognised represents the visibility threshold; colour reticula can also be used (especially of blue-yellow type) to distinguish retinal pathologies.
Other significant visual functionality tests include colour sense examinations (Holmgren""s test, Ishihara""s test) and light sense examinations (visual acuity recovery test following macular halation).
All the tests described are produced in stationary conditions and supply the results relating to the visual performance of a subject at a certain moment. However, it is known that visual performance is greatly influenced by ambient brightness and pupillary diameter, mainly as a consequence of the increase in optic aberrations with the same diameter.
Therefore, conventional apparatuses are incapable of quantifying an identical visual performance, chosen from a range of possibilities at different pupillary opening values basically corresponding to photopic type ambient brightness (full light), mesopic (semi-darkness) and scotopic (dark) conditions.
Therefore the aim of the present invention is to demonstrate a measuring apparatus of visual performance for ophthalmologic examinations, which obviates the above mentioned drawbacks, allowing the various visual functionality tests to be carried out in controlled and variable pupillary opening conditions of a subject, by controlling the ambient brightness and measurement of the pupillary diameter.
Another aim of the present invention is to demonstrate a measuring apparatus of visual performance of a patient which allows the assessment of such performance without the same being, in any way, influenced by the ambient illumination conditions, in which measurement is made, and hence, by the pupillary diameter of the eye being examined.
Another aim of the present invention is to produce an apparatus for measuring visual performance, which allows such performance to be assessed without the it being, in any way, influenced by optic aberrations.
A further aim of the present invention is to produce an apparatus for measuring visual performance which avoids any alteration of the outcome of the visual acuity test.
Not least, the aim of the present invention is to produce an autonomous apparatus for measuring visual performance which reduces the need for operator presence and, at the same time, maintaining essential levels of reliability and flexibility, compared to conventional apparatuses.
Such aims are reached by an apparatus for measuring visual performance according to claim 1, to which reference is made for the sake of brevity.
Advantageously, the apparatus, subject of the present invention, is able to produce the various visual functionality tests of a complete ophthalmologic examination in controlled and variable pupillary opening conditions of the subject being examined, by controlling ambient brightness and measurement of the pupillary size of the above mentioned subject.
It is, in fact, known that visual performance is largely influenced by pupillary diameter, mainly as a consequence of the increase in optic aberrations with the same diameter.
By way of a suitable measurement process, the apparatus is thus able to quantify an identical visual performance, chosen from a range of possibilities, at different pupillary opening values e.g. at 3, 5, 7 mm; values basically corresponding to the photopic, mesopic and scotopic conditions.
The adoption of random character sequences produced by a computer eliminates any chance of the patient memorising them and thus altering the test results.
The implementation of the headset form of apparatus for the patient being examined, accompanied by voice recognition and synthesis circuits, creates a largely autonomous measurement station, which reduces the need for operator intervention.
In addition, the various test results are memorizable and available for printing and statistical processing.
Use of the apparatus is, in short, particularly important in establishing the basic visual performance level in patients undergoing refractive surgery and for comparing its post-surgery development.
Besides, the test is useful for individuals working in extreme light environments, for instance vehicle drivers or airline pilots.